Influence of Material and Testing Parameters on the Lifetime of TBC Systems with MCrAlY and NiPtAl Bondcoats

Song, Peng

Dissertation / PhD Thesis/Book

PreJuSER-19785

Influence of Material and Testing Parameters on the Lifetime of TBC Systems with MCrAlY and NiPtAl Bondcoats

Song, P. (Corresponding author)FZJ*

2011
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich
ISBN: 978-3-89336-783-2

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Schriften des Forschungszentrums Jülich : Energie & Umwelt / Energy & Environment 137, V, 126 S. (2011) = RWTH Aachen, Diss., 2011

Please use a persistent id in citations: http://hdl.handle.net/2128/4566

Abstract: The oxidation behavior of the bond coat is an important factor determining the lifetime of thermal barrier coatings (TBC) in the advanced gas turbine components. In the present work, the effect of various testing parameters, such as hot/cold dwell time, heating/cooling rate, atmosphere composition on the bondcoat oxidation and associated TBC lifetime has been investigated. The range of coating systems included Electron Beam - Physical Vapor Deposited (EB-PVD) and Air Plasma Sprayed (APS) TBC´s with MCrAlY (M = Ni, Co) and NiPtAl- bondcoats of various compositions. The effect of the testing parameters strongly depended on the type and properties of the studied system. The lifetime of EB-PVD TBC systems with conventional MCrAlY and NiPtAl bondcoats forming uniform, flat alumina scales was found to be limited by critical scale thickness, upon which a rapid crack propagation at the scale/bondcoat interface results in macroscopic failure. The lifetime of such systems was found to be affected by factors, which influence the scale growth rate and adherence (in particular by oxygen partial pressure (pO$_{2}$) and water vapor content in the test gas in the case of MCrAlY), whereas the temperature cyclic frequency showed no significant effect. NiPtAl bondcoats showed a superior behavior than the conventional MCrAlY-bondcoats due to slower scale growth rate and better scale adherence. For EB-PVD TBC systems with Zr-doped MCrAlYbondcoats the lifetime is mainly determined by the crack growth rate in the inhomogeneous inwardly growing oxide scales, whereas the lifetime is not dependent on the pO$_{2}$ but rather on the cyclic frequency. For APS TBC systems the bondcoat oxidation is only one of several factors determining the ceramic topcoat lifetime. Therefore the oxide scale adherence is of less importance for lifetime of APS TBCs as compared to EBPVD TBCs. For the former systems, the cracks initiated at the convex asperities of the rough oxide scale / bondcoat interface need to propagate through the TBC to cause macroscopic failure. The rate of crack propagation in the TBC is a critical step, which depends substantially on its microstructural properties. In addition to the TBC-porosity the bondcoat roughness profile is shown to be an important parameter, which to a large extent determines the rate of crack initiation and propagation. Higher Co-content in the bondcoat was found to stabilize its microstructure thereby lowering the CTE-mismatch stress in the ceramic topcoat thus extending the TBC-lifetime. The major drawback of high Co-contents was that such bondcoats are prone to form fast-growing spinel oxides. This effect, which was especially pronounced on rough surfaces could be suppressed by only a minor (few microns) enrichment of Al on the bondcoat surface prior to TBC-deposition produced by heat-treatment in high vacuum. With respect to the effects of experimental parameters it was found that contrary to EB-PVD TBC systems a higher cycle frequency leads to shortening of the APS TBC lifetime, whereas higher water vapor content had no significant influence. The results of the present work indicate that the lifetime of the TBC systems with MCrAlY bondcoats would be shorter than that required for long-term operation (25 000 hours) at the envisaged operating temperature of 1000°C. Under such circumstances using NiPtAl-type of bondcoats or perhaps Pt-modified MCrAlY-bondcoats would be an option to obtain the necessary lifetime extension, which can even justify the high cost of metallic Pt.

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